Metal framing systems for holding glass and various kinds of opaque panels have been in use for some time, both for store front applications and for light and heavy curtain walls. In store fronts and in light curtain walls, a trend has developed in the direction of making the elements of the framing systems as narrow as possible for aesthetic reasons. Typical narrow mullions presently being installed are about one and three fourths inches wide, i.e. the same width as "two-by-four" finished lumber. Problems are encountered in making mullions narrower than this width. First, panel installation (glazing) becomes more difficult because there is less "maneuvering room" in the glazing pockets prior to installation of the glazing gaskets. In addition, the narrowness of the mullion reduces the depth by which the mullion grips the panel (termed "glass bite") if a conventional extrusion profile is employed. Furthermore, the restricted space available in the hollow interior of a narrow mullion makes conventional joint forming techniques with internal brackets difficult to apply to produce a joint between vertical and horizontal mullions having adequate strength.